This application relates generally to fiber optics and more specifically to techniques and devices for monitoring signals in multi-wavelength fiber optic communication channels.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
At each end of a multi-wavelength optical link are wavelength multiplexers and demultiplexers. A wavelength multiplexer (wavelength combiner) combines the light from multiple fibers, each carrying light of different wavelength band, onto the single fiber. A wavelength demultiplexer (wavelength splitter) performs the reverse operation by directing light on the single fiber, which carries light in each of a plurality of wavelength bands, onto separate fibers for each band. Such devices are generally reversible, functioning as a wavelength multiplexer in one direction and as a wavelength demultiplexer in the other direction.
It is necessary in the operation of fiber optic networks to monitor one or more parameters of each of the signal channels in order to detect potential problems and initiate corrective measures. A typical approach is to measure the signal quality of each optical channel at a location where the individual wavelengths have already been demultiplexed into separate fibers, and the optical signals on the separate fibers converted to electrical signals using suitable photodetectors. A portion of the electrical signal is tapped off and communicated to suitable high-speed electronic circuitry for monitoring the desired parameters for that signal""s corresponding wavelength channel. Typical measures of optical signal quality include signal-to-noise ratio, bit error rate, optical power level, and optical wavelength center frequency. Additionally, certain signal protocols (e.g., SONET, which stands for synchronous optical network, and which uses time division multiplexing) provide information within the digital signal stream relating to signal quality. The various techniques for monitoring optical signal quality are well known and are in widespread commercial use.
Thus the wavelength-monitoring circuitry would be deployed in association with the transmission equipment, or at other locations where the signals are already wavelength demultiplexed and converted to electrical signals, such as at a regenerator or optical cross-connect system (OXC). As is known, a regenerator (also known as a repeater) performs optical-to-electrical conversion of each channel, followed by electrical signal amplification, shaping, or other conditioning, followed by electrical-to-optical conversion and subsequent wavelength multiplexing. Similarly, an OXC typically converts the optical wavelengths to electrical signals in order to perform the cross-connection function in the electrical domain. While in the electrical domain, it is possible to monitor the quality of the optical signal transmission. However this approach is limited to systems where expensive optical-to-electronic-to-optical (O-E-O) conversion is used in the transmission path. Newer fiber optic network architectures avoid such O-E-O conversion.
Possibly in recognition of this potential limitation, an alternative prior art approach is to tap a small fraction of the light off the fiber (at any desired location along the link), demultiplex the different wavelength channels onto individual fibers, convert each of the optical signals into electrical signals using suitable photodetectors, and communicate each of the electrical signals to suitable wavelength-monitoring circuitry. However, this technique, while providing additional flexibility, requires the replication of the photodetectors and high-speed electronic circuitry for each wavelength channel, adding cost and complexity to the system.
A third approach is to tap a small fraction of the light off of the fiber, pass the light through and optical train which includes a dispersive element, such as a diffraction grating, and image the individual wavelengths onto individual photodetectors. Like the previous approach, the disadvantage of this technique is the high cost of the replicated photodetectors and related high-speed electronic circuitry.
The present invention provides a cost-effective and versatile technique for monitoring desired signal parameters of multi-wavelength optical links.
In short, a method of monitoring input light having a plurality of spectral bands (wavelength channels) includes the following, carried out for at least two different spectral bands at different times, using a common photodetector and wavelength-monitoring circuit that is coupled to the photodetector: separating one of the spectral bands from the plurality of spectral bands, directing light in only that spectral band to the photodetector, and generating, with the wavelength-monitoring circuit, a signal representing a quality characteristic of a modulated or unmodulated pattern of light in that spectral band.
There is no particular required sequence of monitoring the different bands. For example, each of the spectral bands can be individually and sequentially monitored in round-robin fashion, each of a subset of the spectral bands can be individually and sequentially monitored in round-robin fashion (to provide selective wavelength monitoring), or the monitoring can be ad hoc in response to external requirements. If desired, the optical power of the plurality of spectral bands can be monitored by directing the light in the spectral bands other than the band that has been separated from the plurality to an additional common photodetector and a common power-monitoring circuit.
Apparatus for monitoring input light having a plurality of spectral bands includes an optical train that provides optical paths for routing the spectral bands, a routing mechanism, a photodetector, and a monitoring circuit coupled to said photodetector to provide a signal representing a quality characteristic of a modulated or unmodulated pattern of light impinging on said photodetector. In specific embodiments, the routing mechanism includes a plurality of dynamically configurable routing elements corresponding to the plurality of spectral bands. The optical train may include a dispersive element that intercepts the input light and directs light in respective spectral bands to respective routing elements. Each routing element has a first state causing that routing element to direct its respective spectral band to the photodetector, and a second state causing that routing element to direct its respective spectral band so as not to reach the photodetector.
The apparatus would typically be used with a control circuit that specifies the states of the routing elements. Thus, a given routing element could be commanded to enter its first state, while the remaining routing elements could be commanded to enter their respective second states, thereby resulting in an output signal from the monitoring circuit that represented the quality characteristic of the spectral band associated with the given routing element.